This invention relates to a laser scanning device for calculating a distance to a target object by projecting a laser beam and measuring the intensity of reflected light.
In order to prevent traffic accidents involving automobiles, it is becoming a common practice in recent years to make use of optical distance measuring apparatus having a laser scanning device to use a near infrared laser beam to scan the front and to detect the presence or absence of an object in front (such as a front going vehicle, an obstacle or a pedestrian) and to measure its distance by receiving its reflected light.
FIG. 10 shows the structure of a laser scanning device. Numeral 101 indicates a distance measuring part serving to cause a light projecting part 102 to project a laser beam. Reflected light of the beam projected from the light projecting part 102 is received by a light receiving element 103 and is thereby converted into an electrical signal according to the intensity of the received light. This electrical signal is amplified by an amplifier 104 and then inputted to an intensity recording part 105 where the intensity of the received light is recorded. As shown in FIG. 11A, the intensity recording part 105 serves to detect a peak in the intensity of the received light along a time axis and outputs the timing for receiving light to the distance measuring part 101. The distance measuring part 101 measures the distance to an object from the delay time measured from the timing of light projection from the light projecting part 102 and the timing of receiving light by the intensity recording part 105.
In the above, the intensity recording part 105 may be adapted to represent the light intensity in terms of specified steps (such as 0-255) and, if a signal exceeding a recordable maximum intensity (MAX) is inputted, to record such a saturated situation by showing the intensity to have that maximum intensity (MAX), as shown in FIG. 11B. In such a situation, it is no longer possible to detect a peak in the intensity. In other words, prior art devices of this type had the problem of not being able to measure the distance of an object in front or to obtain an erroneous distance.
In view of this problem, Japanese Utility Model Publication Jikkai 5-23176, for example, proposed to reduce the gain of the amplifier upon detecting a saturated condition of the electrical signal. Japanese Patent Publication Tokkai 7-146368 disclosed a radar device adapted to alternately measure at a maximum gain and a reduced gain so as to be able to detect both objects with strong reflected light and objects with weak reflected light. Japanese Patent Publication Tokkai 8-220234 disclosed a method of controlling the gain for each area such that the received intensity is stable and to the make this central angle of scan in the region with this stabilized reception intensity as the object of measurement.
Each of the technologies described above requires, however, that the intensity of received light be measured preliminarily in the target area so as to thereafter control the gain such that saturation will not occur by the received signal. If such preliminary measurements are necessary for controlling the gain, extra time will have to be spent for this purpose and the time efficiency of the measurement is adversely affected. If the gain control is to be carried out frequently (such as every time a measurement is made), an accurate measurement of distance may become possible but it will take twice as long to do so with the preliminary measurement and the actual measurement. This means that the efficiency drops to 50%.